Rebels opposed to Muammar Gaddafi calling for more western intervention in the besieged Libyan city of Misrata yesterday. Photograph: Odd Andersen/AFP/Getty Images

The British government has come under intense pressure over its response to the crisis in Libya as ministers prepared to dispatch a team of military officers to advise rebels fighting Muammar Gaddafi's forces and the RAF stepped up air strikes.

Nato commanders said the alliance was extending Nato's targets in Libya to include small satellite communications systems and telephone exchanges in strikes described by defence officials as marking a clear "shift" in targeting policy. MPs expressed deep concern about mission creep.

The UN appealed for a ceasefire in Misrata, saying at least 20 children had been killed in attacks by government forces on rebel-held parts of the city, but senior Nato officers admitted air strikes could do little on their own to prevent a worsening crisis there.

Officials from countries engaged in the bombing campaign made it clear the situation is becoming increasingly difficult. Military action is not securing their goal, the end of the Gaddafi regime, while more direct intervention would be unpopular at home and might breach UN security council resolutions on Libya.

"It is a step to prolong the confrontation, there is no doubt about that," foreign minister Abdul Ati al-Obeidi said. "Sending military personnel will encourage the other side to be more defiant." Libya's deputy foreign minister, Khaled Kayim, said the Anglo-French decision was a "clear violation" of the UN security council remit and will prolong the crisis engulfing the country.

William Hague, the foreign secretary, said a number of "experienced military officers" would be sent to Benghazi to advise the rebels on intelligence-gathering, logistics and communications. About 10 British and 10 French officers will make up a combined team run by a joint headquarters.

The advisers "will enable the UK to build on the work already being undertaken to support and advise the NTC [National Transitional Council] on how to better protect civilians", Hague said. "In particular they will advise the NTC on how to improve their military organisational structures, communications and logistics, including how best to distribute humanitarian aid and deliver medical assistance."

British defence officials stressed the team would consist of advisers, rather than trainers. The officers will be equipped with small arms for their own protection but would not arm the rebels. There are no plans for the team to go to Misrata, the officials added.

General Lord Dannatt, a former head of the British army, said the decision to send military advisers to Benghazi was "an entirely logical further step to achieve legitimate aims".

Referring to last week's joint statement by David Cameron, Barack Obama and Nicolas Sarkozy that it would be an "unconscionable betrayal" if Gaddafi were allowed to remain in power, Dannatt added: "Some will always say 'mission creep', but [Britain should] interpret the UN mandate broadly to avoid mission collapse."

But the decision provoked a political row at home. David Davis, a former Conservative home secretary, said: "A lot of people will see this as mission creep, some of us as an inevitable outcome."

The ex-Liberal Democrat leader Sir Menzies Campbell said: "Sending advisers for a limited purpose is probably within the terms of Resolution 1973, but it must not be seen as a first instalment of further military deployment. Vietnam began with a US president sending military advisers. We must proceed with caution."

The Labour MP David Winnick said: "However much one despises the brutality of the Gaddafi clan which rules Libya, the fact remains that there is a danger of mission creep. There is a civil war in Libya and this is a big escalation of Britain's involvement."

The six European countries in the bombing campaign – led by the French and the British, and including the Danes, Norwegians and Belgians – along with the Canadians, appear to have closed the gap in the bombing capacity caused by the withdrawal of the US more than a fortnight ago.

Officials said France had taken on the lead role, moving the Charles De Gaulle aircraft carrier closer to the Libyan coast and placing the aircraft on board under Nato command.

Officials pointed specifically to an attack near Tripoli that destroyed the communications facilities of Gaddafi's crack 32nd brigade, regarded as the most formidable unit in the regime's military. The aim was to cut the regime's supply lines and disrupt Gaddafi's ability to direct his forces.

Alain Juppe, the French foreign minister, said he remained "absolutely opposed" to troop deployment on the ground. But he acknowleged: "What we have perhaps underestimated is Muammar Gaddafi's capacity to adapt."

The British submarine HMS Triumph, is understood to have fired cruise missiles at Libyan targets over the past two days.

It emerged that last Friday's meeting of Nato foreign ministers supplied the go-ahead for more intensive bombing and securing more aircraft and weaponry.

"Gaddafi is not able to use his forces as he wants. He's not able to fight the way he wants," said Brigadier General Mark van Uhm, Nato's chief of allied operations. But "nothing indicates that Gaddafi has any intention of disengaging. His forces continue to use heavy weapons".

Nato officers admitted they could have little impact on the urban warfare in Misrata. "There's a limit to what can be achieved by air power to stop fighting in a city," said Van Uhm.

Lieutenant-General Charles Bouchard, the Canadian commander of the air campaign, told German radio: "It's a knife fight in a phone booth and it's very difficult to get in the middle of that."

Aid groups say food, medicines and other basic items are in short supply in the city, and tens of thousands of casualties and foreign workers are waiting at the port to be evacuated.

• This article was amended on 20 April 2011. The original referred to the six European countries in the bombing campaign – led by the French and the British and also including the Canadians, Danes, Norwegians, and Belgians. This has been corrected.